This invention relates to a drive and brake circuit for a speed controlled DC motor, and particularly to circuits for driving and braking motors that power or impart motion to rolling vehicles.
Such circuits often apply current to the motor from a DC source through a choke and a DC regulator or controller. A brake resistance and a first diode is generally connected across the motor field winding. A circuit arrangement including switches shifts the motor between a drive mode and a brake mode. In a drive mode the armature is connected in parallel with a field winding and a series rectifier. In the braking mode the armature is connected across a brake resistance and the series rectifier. An example of such a circuit is disclosed in German patent publication P 23 19 898.
A circuit of this type has the advantage that a continuous weakening of the field occurs with increasing armature voltage, both in the drive and braking modes. Also, the brake resistance need not be stepped, that is, galvanic switching apparatuses are not necessary for varying the brake resistance in response to the voltage which the motor supplies when it acts as a generator. However, such known circuits do not permit regenerative braking, i.e., feeding energy back into the power system that supplies the motor. Such regenerative braking is valuable in a traffic network of electric-motor operated vehicles because it reduces the total energy consumed by the traffic network utilizing such motors.
However, a disadvantage of such regenerative braking circuits is that the brake is not always available when the power system fails, or when there are no vehicles capable of absorbing the energy. Thus regenerative braking cannot be used. In contrast with the concept of a regenerative braking system, the aforementioned circuit using the brake resistance is independent of the power system.
An object of the invention is to improve drive and brake systems.
Another object of the invention is to adapt the aforementioned known resistance brake circuit as a regenerative brake.
According to the invention, these objects are attained, in whole or in part, by making the rectifier controllable and connecting the armature alternately across the source when the source voltage is sufficiently low to indicate that the network is capable of absorbing energy, and across the brake resistor, when the source voltage exceeds a predetermined value and thereby indicates that the network is incapable of absorbing energy.
According to another feature of the invention, the rectifier is controllable and a second diode is arranged between the extinguishing diode of the DC control device and one terminal of the resistor while, during braking operation, a third diode is arranged between one terminal of the field winding and one terminal of the choke. By virtue of these features, the system performs dynamic braking both regeneratively and non-regeneratively, or resistively. While driving, the braking circuit's controllable rectifier, in the form of a thyristor, operates as a switching member for the free running current of the field winding. While driving the motor this circuit behaves substantially like the aforementioned circuit disclosed in German patent publication No. 2,319,898.
According to another feature of the invention, suitable means ignite the thyristor during the drive operation a predetermined time interval after the ignition pulse of a thyristor in the DC controller. This provides a firing pulse after each inactive period of the controller. Such a pulse is necessary because a thyristor is provided instead of an armature-free-running diode in the aforementioned German patent publication No. 2,319,898.
According to another feature of the invention, suitable means set the time interval between the firing pulse of a reversing thyristor and the firing pulse of the main thyristor in dependence upon the armature current. This utilizes the fact that the inactive period of the DC controller or regulator depends upon the armature current.
According to another embodiment of the invention, means fire the thyristor during braking, only when a predetermined minimum voltage is applied across a capacitor. Suitable means apply the firing pulses of the thyristor after a controllable time interval from the firing pulse of the extinguishing thyristor in the DC regulator.
According to another feature of the invention, means respond to a sensor across the voltage applied to the capacitor for deriving the time interval of the firing pulses of the main thyristor from the firing pulses of the extinguishing thyristor.
According to another embodiment of the invention, a fourth diode extends between the input of the DC controller and the line joining one terminal of the field winding of the drive motor with the first diode. The second diode conducts in the forward direction between the anode of the thyristor and the cathode of the extinguishing diode in the DC controller.
Moreover, according to a preferred embodiment of the invention, the brake resistor is provided with a tap which is connected to one terminal of the field winding. One end of a switch is connected to the line joining the first diode with the terminal of the field winding, and a second end is connected with the first terminal of the resistor. A second switch includes one end connected to the tap of the resistor joined to the second terminal of the field winding, and a second end is connected with the first diode as well as the second terminal of the resistor. A third switch connects the current source through the choke with the input of the DC control device or regulator. A fourth switch is connected between the ground potential of the capacitor and the conducting line. During drive operation the first switch, the second switch and the third switch as well as the fourth switch are closed. During braking, the aforementioned switches are opened and a supplementary switch connects the output of the DC regulator to ground potential.
During braking, the DC regulator is connected parallel to the armature. The third diode and the DC controller connects the field parallelto the armature. Thus, one segment of the resistor constitutes a compound resistance in the field free running current path. The first diode operates as a decoupling diode to the power network and the thyristor serves as a switch for connecting the brake resistor. At the same time the second diode produces a connection to the extinguishing circuit of the DC regulator. The latter permits extinction of the thyristor in the brake resistor circuit simultaneously with the main thyristor of the DC regulator.
These and other features of the invention will be pointed out in the claims forming portion of this specification. Other advantages of the invention will become evident from the following detailed description when read in light of the accompanying drawing.